I can't get HD Voice/VoLTE working on my unlocked OnePlus 6T

Hello,

I recently noticed that the OnePlus 6T was added to the list of devices that work with VoLTE. I tried enabling VoLTE on my (unlocked) phone, but nothing changed (calls still switch over to 3G). I've reset my network settings and confirmed that my SIM card supports HD voice (the 9th and 10th digits of my SIM card number are "27"), but nothing has worked. Is there something I'm missing?

Thanks!